Carbon is known to have five unique crystalline structures, including diamond, fullerene (0-D nano graphitic material), carbon nano-tube (1-D nano graphitic material), graphene (2-D nano graphitic material), and graphite (3-D graphitic material).
The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material.
A graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. An isolated, individual graphene sheet is commonly referred to as single-layer graphene. A stack of multiple graphene planes bonded through van der Waals forces in the thickness direction is commonly referred to as a multi-layer graphene, typically having 2-300 layers or graphene planes, but more typically 2-100 graphene planes. Single-layer graphene and multi-layer graphene sheets are collectively called “nano-scaled graphene platelets (NGPs).” Our research group pioneered the development of graphene materials and related production processes: (1) B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006), application submitted in October 2012]; (2) B. Z. Jang, et al. “Process for Producing Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004); and (3) Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25, 2006).
NGPs are typically obtained by intercalating natural graphite flakes with a strong acid and/or oxidizing agent to obtain a graphite intercalation compound (GIC), as illustrated in FIG. 1. This is most often accomplished by immersing natural graphite flakes in a mixture of sulfuric acid, nitric acid (an oxidizing agent), and another oxidizing agent (e.g. potassium permanganate or sodium chlorate). The resulting WC is actually some type of graphite oxide particles. This GIC is then repeatedly washed and rinsed in water to remove excess acids, resulting in a graphite oxide suspension or dispersion which contains discrete and visually discernible graphite oxide particles dispersed in water. There are two processing routes to follow after this rinsing step:
Route 1 involves removing water from the suspension to obtain “expandable graphite,” which is essentially dried GIC or dried graphite oxide particles. Upon exposure of expandable graphite to a temperature in the range of 800-1050° C. for approximately 30 seconds to 2 minutes, the GIC expands by a factor of 30-300 to form a “graphite worm,” which is a collection of exfoliated, but largely unseparated or interconnected graphite flakes. In Route 1A, these graphite worms (exfoliated graphite or “networks of interconnected/non-separated graphite flakes”) can be re-compressed to obtain flexible graphite sheets that typically have a thickness in the range of 0.125 mm (125 μm)-0.5 mm (500 μm). These flexible graphite (FG) sheets are used as a seal material and a heat spreader material, but exhibiting a maximum in-plane thermal conductivity of up to 600 W/mK (typically <300 W/mK) and in-plane electrical conductivity no greater than 1,500 S/cm. In Route 1B, the exfoliated graphite is subjected to mechanical shearing (e.g. using an ultrasonicator, high-shear mixer, air jet mill, or ball mill) to form separated single-layer and multi-layer graphene sheets (collectively, NGPs), as disclosed in our U.S. application Ser. No. 10/858,814.
Route 2 entails ultrasonicating the graphite oxide suspension for the purpose of separating individual graphene oxide sheets from graphite oxide particles. This is based on the notion that the inter-graphene plane separation has been increased from 0.335 nm in natural graphite to 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakening the van der Waals forces that hold neighboring planes together. Ultrasonic power can be sufficient to further separate graphene plane sheets to form separate, isolated, or discrete graphene oxide (GO) sheets. These graphene oxide sheets can then be chemically or thermally reduced to obtain “reduced graphene oxides” (RGO) typically having an oxygen content of 2-10% by weight, more typically 25% by weight.
For the purpose of defining the claims of the instant application, NGPs include single-layer and multi-layer graphene or reduced graphene oxide with an oxygen content of 0-10% by weight, more typically 0-5% by weight, and preferably 0-2% by weight. Pristine graphene has essentially 0% oxygen.
It may be noted that flexible graphite sheets (obtained by re-compression of exfoliated graphite or graphite worms) exhibit a relatively low thermal conductivity (<600 W/mK as recited above). Flexible graphite sheets are also of low strength and poor structural integrity. The high tendency for flexible graphite sheets to get torn apart makes them difficult to handle in the process of integrating them in a microelectronic device.
Similarly, the NGPs, when packed into a film or paper sheet of non-woven aggregates, exhibit a thermal conductivity higher than 1,000 W/mK only when the film or paper is cast and pressed into a sheet having a thickness lower than 10 μm, and higher than 1,500 W/mK only when the film or paper is cast and pressed into a sheet having a thickness lower than 1 μm. This is reported in our earlier U.S. patent application Ser. No. 11/784,606 (Apr. 9, 2007). However, ultra-thin film or paper sheets (<10 μm) are difficult to produce in mass quantities, and difficult to handle when one tries to incorporate these thin films as a heat spreader material during the manufacturing of microelectronic devices. Further, thickness dependence of thermal conductivity (not being able to achieve a high thermal conductivity at a wide range of film thicknesses) is not a desirable feature.
Our earlier application (Ser. No. 11/784,606) further disclosed a mat, film, or paper of NGPs infiltrated with metal, glass, ceramic, resin, and CVD graphite matrix material. Later on, Haddon, et al (US Pub. No. 2010/0140792, Jun. 10, 2010) also reported NGP thin film and NGP-polymer composites for thermal management applications. The processes used by Haddon et al to produce NGPs are identical to those disclosed much earlier by us (Jang, et al. U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004)). Balandin, et al (US Pub. No. 2010/0085713, Apr. 8, 2010) also disclosed a graphene layer produced by CVD deposition or diamond conversion for heat spreader application. More recently, Kim, et al (N. P. Kim and J. P. Huang, “Graphene Nanoplatelet Metal Matrix,” US Pub. No. 2011/0108978, May 10, 2011) reported metal matrix infiltrated NGPs. However, metal matrix material is too heavy and the resulting metal matrix composite does not exhibit a high thermal conductivity.
Another prior art material for thermal management application is the pyrolitic graphite film. The lower portion of FIG. 1 illustrates a process for producing prior art graphitic films or sheets. The process begins with carbonizing a polymer 46 at a carbonization temperature of 500-1,000° C. for 2-10 hours to obtain a carbonized material 48, which is followed by a graphitization treatment at 2,500-3,200° C. for 5-24 hours to form a graphitic film 50. This is a slow, tedious, and energy-intensive process. Carbonization of certain polymers (e.g. polyacrylonitrile) involves the emission of toxic species.
Thus, it is an object of the present invention to provide a highly conductive GO gel-bonded NGP composite thin-film structure (and related production processes) that exhibits a thermal conductivity greater than 600 W/mK, typically greater than 800 W/mK, more typically greater than 1,500 W/mK (even when the film thickness is greater than 10 μm), and most preferably and often greater than 1,700 W/mK.
It is another object of the present invention to provide an NGP-GO composite thin-film sheet that exhibits a relatively thickness-independent thermal conductivity.
Still another object of the present invention is to provide a GO-bonded pristine graphene composite thin film that exhibits exceptional thermal and electrical conductivity properties.
It is a further object of the present invention to provide an NGP-GO composite thin-film sheet that is lightweight and exhibits a relatively high strength or structural integrity.
It is yet another object of the present invention to provide a highly conductive NGP-GO composite thin-film sheet (and related processes) wherein the in-plane thermal conductivity is greater than 600 W/mK (preferably and typically greater than 1,000 W/mK) and in plane electrical conductivity is greater than 2,000 S/cm (preferably and typically >3,000 S/cm), and/or a tensile strength greater than 10 MPa (preferably and typically >40 MPa).
It is another object of the present invention to provide a highly thermally conductive NGP-GO composite thin-film sheet that can be used for thermal management applications; e.g. for use as a heat spreader in a microelectronic device (such as mobile phone, notebook computer, and tablet), flexible display, light-emitting diode (LED), power tool, computer CPU, and power electronics. We are filing separate patent applications to claim the various products or applications of the presently invented NGP-GO composite thin-films.